In the manufacture and use of abrasive and cutting tools such as grinding wheels, drill bits and saw blades, cubic boron nitride and diamond have been recognized as excellent abrasive materials. Their dense crystalline structure and strong covalent bonds provide high hardness, high abrasion and advantageous breakdown characteristics when used in grinding and cutting applications. These materials are also excellent thermal conductors and efficiently transfer heat generated during cutting and grinding operations from a tool's cutting edge. Cubic boron nitride and diamond are often used in the form of sized particles bonded by a resin or metal matrix in tools such as, for example, grinding wheels. They are also used as clusters of crystals bonded together, with or without the aid of a bonding medium, in the form of a solid mass or compact. This compact can be affixed to a cutting tool or formed directly on a substrate.
To be effective in grinding tool applications, the particles must be of a size sufficiently large to protrude from the cutting surface of the tool intended. Obtaining suitably sized particles is difficult. Cubic boron nitride (CBN) and industrial diamonds are synthesized from materials of an alternate crystalline structure, i.e., hexagonal boron nitride and graphite, respectively. These materials are made under conditions of high pressure and high temperature, with or without the aid of a catalyst. See U.S. Pat. Nos. 4,289,503; 4,188,194; and 3,918,219. Polycrystalline particles of a desired size can be obtained by a direct conversion process using starting materials (HBN) of a similar but larger size under conditions of high temperature and pressure. Alternatively, large polycrystalline masses can be formed from diamond or CBN powders with the aid of a catalyst under conditions of high temperature and pressure for periods in excess of 15 minutes and as long as one hour. These polycrystalline masses are then milled to particles of a desired mesh size.
Copending application Ser. No. 07/365,883, entitled "Process for Making Cubic Boron Nitride from Coated Hexagonal Boron Nitride and Abrasive Particles and Articles Made Therefrom", filed June 14, 1989, and assigned to the same assignee as the present invention, describes a method wherein hexagonal boron nitride particles are coated with a layer that prevents oxide contamination prior to conversion to polycrystalline CBN. The protective layer is bonded chemically within the resultant polycrystalline CBN in the form of a compact. This compact can be milled to abrasive particles of a desired size with controlled breakdown characteristics. However, this process utilizes high temperatures and pressures for converting the HBN. Furthermore, while application Ser. No. 07/365,883 discloses that the CBN abrasive particles obtained can be sintered into another multigrain mass, some of the original coating is said to be lost in breaking up the CBN compact.
Making abrasive particles of a desired size by prior art techniques is costly in that they require significant expenditures of energy and place considerable demands on high pressure equipment due to their long cycle times. It is desirable to shorten the cycle times necessary to form suitably sized abrasive particles and it is also desirable to operate at lower temperatures and pressures.
A method for producing aggregate abrasive grains is described in U.S. Pat. No. 4,024,675, wherein a mixture of abrasive powders, sinterable metal alloy powders (Ca, Sn, Ni, Co, Fe) and an adhesion active agent (catalyst) are sintered to produce a porous cake with voids which is subsequently crushed to form abrasive particles of a desired size. While this process does not utilize high pressures and temperatures for extended periods of time, the particles produced do not exhibit high toughness and thermal stability since the metal alloys are not coated and bonded to the abrasive. In addition, manipulating the breakdown characteristics of the resultant particles is not described and may not be possible.
As can be seen from the foregoing, it would be advantageous to form tough, thermally resistant abrasive particles of CBN and diamond by techniques which utilize high pressure/high temperature equipment more efficiently or do not require such an apparatus at all. It would be particularly advantageous to provide such multigrain abrasive particles without the use of a catalyst (adhesive active agent) when sintering techniques are used. It would also be advantageous to produce multigrain abrasive particles with controlled breakdown characteristics by methods which do not require long exposure to high temperatures and pressures.
Various metals and alloys of metals are known to enhance the bond retention of abrasive particles in the matrices of abrasive tools such as grinding wheels and similar agglomerated aggregates. For example, coatings of nickel are applied to abrasive particles used in grinding wheels or similar aggregates bonded with a resin or metal matrix. Suitable coating techniques known in the art include electrolytic deposition, electroless deposition, sputtering, vacuum deposition and heat treatment of a metal/particle mixture. See, Pipkin, U.S. Pat. No. 4,399,167. Until now, coated granules and powders have not been used efficiently to provide abrasive particles of a larger size with high toughness and thermal resistance.